Neutral-grounded structure for delta-connected windings and method thereof

ABSTRACT

A neutral-grounded structure and a method for delta-connected windings are provided. The delta-connected windings include a first phase winding, a second phase winding, and a third phase winding. A grounding winding is disposed on at least one of the phase windings. The first end of the grounding winding is connected to a grounded point which is utilized as a neutral point. The second end of the grounding winding is connected to a tap of one of the other two phase windings. The voltage phase displacement between the grounding winding and the phase winding corresponding to the grounding winding is 180 degrees. According to the above-mentioned neutral-grounded structure, all the three phase-to-ground voltages are the same when the three phase voltages are balanced. Accordingly, the present invention can solve correlated problems due to differences of the three phase-to-ground voltages of delta-connected windings with conventional grounding methods.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to winding connection, and moreparticularly to a neutral-grounded structure for delta-connectedwindings and method thereof.

BACKGROUD OF THE INVENTION

The connection methods of a three phase windings can be adelta-connected or a wye-connected. In comparison, the desireddielectric strength of the wye-connected windings is lower. However, thegrounded wye-connected windings have defects of providing a path forzero-sequence current, third harmonic current, and 3× harmonic currentsto pass through. Accordingly, the grounded wye-connected windings areunfavorable to efficiency and quality in a power distribution system.Although the delta-connected windings do not have the defects ofproviding a path for zero-sequence current and harmonic currents to passthrough, the desired dielectric strength of the delta-connected windingsis higher than that of the wye-connected windings. As a result, when thedelta-connected windings and the wye-connected windings are applied inthe same power distribution system, the dielectric strength of thedelta-connected windings has to be higher than that of the wye-connectedwindings. The delta-connected windings are also called Δ-connectedwindings. Generally speaking, conventional grounding methods in thedelta-connected windings include a line-grounded method and a mid-phasegrounded method. The line-grounded method is also called acorner-grounded method. FIG. 1 illustrates a diagram of theline-grounded method in the delta-connected windings. In theline-grounded method, one corner of the delta-connected windings isgrounded. A phase line c is drawn forth and grounded in FIG. 1.Accordingly, the voltage of the phase line c is in the ground potential,and the voltage between each of the other two phase lines and thegrounded point n is a line voltage. FIG. 2 illustrates a diagram of themid-phase grounded method in the delta-connected windings. In themid-phase grounded method, a mid-tap in one of the delta-connectedwindings is drawn forth to a grounded point n. As shown in FIG. 2, themid-tap is in the middle of the winding between the phase line a and thephase line c, and the mid-tap is drawn forth and connected to thegrounded point n. As a result, the voltage between the phase line a andthe grounded point n or between the phase line c and the grounded pointn is half of the line voltage, and the voltage between the phase line band the grounded point n is √{square root over ( )}3/2 times of the linevoltage.

From the above-mentioned, the conventional grounding methods in thedelta-connected windings lead to differences of the threephase-to-ground voltages. And many problems are thus derived. Forexample, when a coil of a contactor in a motor control circuit isconnected to the improper phase lines of the delta-connected windings,the coil might be broken due to effects of grounded points in theconventional grounding methods.

SUMMERY OF THE INVENTION

The primary objective of the present invention is to provide aneutral-grounded structure for delta-connected windings. Thedelta-connected windings include a first phase winding, a second phasewinding, and a third phase winding. Each of the first phase winding, thesecond phase winding, and the third phase winding has a plurality ofturns, e.g. N turns. A grounding winding is disposed on at least one ofthe first phase winding, the second phase winding, and the third phasewinding. The grounding winding has turns that are (⅓)N turns of one ofthe phase windings. The first end of the grounding winding is connectedto a grounded point which is utilized as a neutral point. The second endof the grounding winding is connected to a tap of one of the other twophase windings. The tap is at (⅓)N turns of one of the other two phasewindings. The voltage phase displacement between the grounding windingand the phase winding corresponding to the grounding winding is 180degrees.

Another objective of the present invention is to provide aneutral-grounded structure for delta-connected windings. Thedelta-connected windings include a first phase winding, a second phasewinding, and a third phase winding. Each of the first phase winding, thesecond phase winding, and the third phase winding has a plurality ofturns, e.g. N turns. A grounding winding group is disposed on at leastone of the first phase winding, the second phase winding, and the thirdphase winding. The grounding winding group includes at least twogrounding windings which are connected in parallel. Each of thegrounding windings has turns that are (⅓)N turns of one of the phasewindings. The first end of the grounding winding group is connected to agrounded point which is utilized as a neutral point. The second end ofthe grounding winding group is connected to a tap of one of the othertwo phase windings. The tap is at (⅓)N turns of one of the other twophase windings. The voltage phase displacement between the groundingwinding group and the phase winding corresponding to the groundingwinding is 180 degrees.

Another objective of the present invention is to provide aneutral-grounded method for delta-connected windings. Thedelta-connected windings include a first phase winding, a second phasewinding, and a third phase winding. Each of the delta-connected windingshas a plurality of turns, e.g. N turns. The neutral-grounded methodincludes the following steps. A grounding winding is disposed on atleast one of the first phase winding, the second phase winding, and thethird phase winding. The grounding winding has turns that are (⅓)N turnsof one of the phase windings. The first end of the grounding winding isconnected to a grounded point which is utilized as a neutral point. Thesecond end of the grounding winding is connected to a tap of one of theother two phase windings. The tap is at (⅓)N turns of one of the othertwo phase windings. The voltage phase displacement between the groundingwinding and the corresponding phase winding is 180 degrees.

According to the neutral-grounded structure for the delta-connectedwindings and method thereof, voltage differences between each of thephase line and the neutral point are the same. When a three-phase poweris fed to the phase windings, the three phase-to-ground voltages are thesame. As a result, problems of differences of the three phase-to-groundvoltages can be solved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a diagram of the line-grounded method in thedelta-connected windings;

FIG. 2 illustrates a diagram of the mid-phase grounded method in thedelta-connected windings;

FIG. 3 illustrates a diagram of disposing one grounding winding on onephase winding according to a neutral-grounded structure fordelta-connected windings of the present invention;

FIG. 4 illustrates a practical circuit according to the neutral-groundedstructure in FIG. 3;

FIG. 5 illustrates a voltage phasor diagram corresponding to theneutral-grounded structure for the delta-connected windings in FIG. 3;

FIG. 6 illustrates a diagram of disposing two grounding windings on twophase windings according to the neutral-grounded structure for thedelta-connected windings;

FIG. 7 illustrates a diagram of disposing three grounding windings onthree phase windings according to the neutral-grounded method for thedelta-connected windings of the present invention;

FIG. 8 illustrates a diagram of disposing six grounding windings to theneutral-grounded structure for the delta-connected windings of thepresent invention;

FIG. 9 illustrates a voltage phasor diagram corresponding to theneutral-grounded structure for the delta-connected windings in FIG. 8;

FIG. 10 illustrates a diagram of disposing a grounding winding group onone phase winding; and

FIG. 11 illustrates a flow chart showing a neutral-grounded method fordelta-connected windings.

DETAILED DESCRIPTION OF THE INVENTION

Please refer to FIG. 3. FIG. 3 illustrates a diagram of disposing onegrounding winding on one phase winding according to a neutral-groundedstructure for delta-connected windings of the present invention. FIG. 4illustrates a practical circuit according to the neutral-groundedstructure in FIG. 3. The practical circuit is a transformer circuit.Primary side of the transformer circuit includes a first phase windingof primary side 20, a second phase winding of primary side 22, and athird phase winding of primary side 24. Connectors H₁, H₂ of each of thephase windings are wye-connected or delta-connected to a three-phasepower (not shown). Secondary side of the transformer circuitcorresponding to primary side of the transformer circuit includes afirst phase winding 30, a second phase winding 32, and a third phasewinding 34. The three phase winding 30, 32, 34 are delta-connected. Thatis, a connector X₃ of the first phase winding 30 is connected to aconnector X₁ of the second phase winding 32. A connector X₃ of thesecond phase winding 32 is connected to a connector X₁ of the thirdphase winding 34. A connector X₃ of the third phase winding 34 isconnected to a connector X₁ of the first phase winding 30. Each of thephase windings in secondary side has N turns. In the neutral-groundedstructure of the present invention, a grounding winding 36 is disposedon at least one of the three phase windings 30, 32, 34 in secondaryside. In the present embodiment, the grounding winding 36 is disposed onthe third phase winding 34. The grounding winding 36 has (⅓)N turns. Inorder to clarify FIG. 3, the grounding winding 36 is illustrated alignedin parallel to the third phase winding 34. This represents that thegrounding winding 36 is disposed on the third phase winding 34. Thefollowing figures are similar to FIG. 3 and are not repeated. Thegrounding winding 36 has a first end and a second end. The first end ofthe grounding winding 36 is connected to a grounded point which isutilized as a neutral point n. The second end of the grounding winding36 is connected to a tap of one of the other two phase windings 30, 32.The tap is at (⅓)N turns of one of the other two phase windings 30, 32.The tap is generated by drawing forth from (⅓)N turns of one of theother two phase windings 30, 32. In the present embodiment, the secondend of the grounding winding 36 is connected to the tap of (⅓)N turns ofthe first phase winding 30. The voltage phase of the grounding winding36 and the voltage of the corresponding phase winding is out of phase.The phase winding corresponding to the grounding winding 36 is the thirdphase winding 34. In the present embodiment, if a voltage of the thirdphase winding 34 is Vca, a voltage of the grounding winding 36 must beVca so that the three phase-to-ground voltages are the same. Theabove-mentioned term “out of phase” means that a voltage phasedisplacement between the grounding winding 36 and the third phasewinding 34 is 180 degrees.

As shown in FIG. 4, if the voltage between two of the phase lines a, b,c respectively is V, each of the voltages between each of the phaselines a, b, c respectively and the neutral point n is V/√{square rootover ( )}3. That is, each of the phase voltages is V/√{square root over( )}3. Accordingly, differences of each of the voltages between thephase lines a, b, c respectively and the grounded point in theconventional grounding methods can be improved. In another aspect, theoriginal phase voltage is V and is now reduced to V/√{square root over ()}3. A requirement for dielectric strength of equipment is thus reduced.This represents that the delta-connected windings can apply to higherphase-to-ground voltage than before.

Please refer to FIG. 5. FIG. 5 illustrates a voltage phasor diagramcorresponding to the neutral-grounded structure for the delta-connectedwindings in FIG. 3. When a three-phase power fed to the delta-connectedwindings is balanced, each of the voltages between each of the phaselines a, b, c respectively and the neutral point is the same. As aresult, differences of each of the phase-to-ground voltages in theconventional grounding methods can be improved. It is noted that avoltage phasor (⅓) Vac of the grounding winding 36 is illustrated inparallel to a voltage phasor Vca of the third phase winding 34. Thisrepresents that the grounding winding 36 is disposed on the third phasewinding 34. The voltage phasor (⅓)Vac of the grounding winding 36 startsfrom the end of a voltage phasor (⅓)Vca of the first phase winding 30.This represents that the second end of the grounding winding 36 isconnected to the tap that is at (⅓)N turns of the first phase winding30. The vector arrow of the voltage phasor (⅓)Vca is opposite to thevector arrow of the voltage phasor Vac. This represents that the voltagephase displacement between the voltage phasor (⅓)Vca and the voltagephasor Vac is 180 degrees.

For considerations of reliability, the grounding windings can bedisposed on two phase windings at the same time. FIG. 6 illustrates adiagram of disposing two grounding windings on two phase windingsaccording to the neutral-grounded structure for the delta-connectedwindings. In the present embodiment, two grounding windings 38, 40 aredisposed on the third phase winding 34 and the first phase winding 30,respectively. Each of the grounding windings 38, 40 has (⅓)N turns. Thefirst end of the grounding winding 38 is connected to the groundedpoint. The grounded point is a neutral point n. The second end of thegrounding winding 38 is connected to a tap at (⅓) number of turns of thefirst phase winding 30, i.e. (⅓)N turns of the first phase winding 30.The first end of the grounding winding 40 is connected to the groundedpoint. That is, the first end of the grounding winding 40 is connectedto the first end of the grounding winding 38. The second end of thegrounding winding 40 is connected to a tap at (⅓)N turns of the secondphase winding 32. The grounding windings 38, 40 should have appropriatevoltage phases. When a three-phase power fed to the delta-connectedwindings is balanced, each of the voltages between each of the phaselines a, b, c respectively and the neutral point n is the same. Theappropriate voltage phases of the grounding windings 38, 40 are the sameas that described in FIG. 3 and omitted herein.

Please refer to FIG. 7. FIG. 7 illustrates a diagram of disposing threegrounding windings on three phase windings according to theneutral-grounded method for the delta-connected windings of the presentinvention. In the present embodiment, three grounding windings 42, 44,46 are disposed on the phase windings 30, 32, 34, respectively. Each ofthe grounding windings has (⅓)N turns. The first end of the groundingwinding 42, the first end of the grounding winding 44, and the first endof the grounding winding 46 are connected to a grounded point. Thegrounded point is a neutral point n. The second end of the groundingwinding 42 is connected to a tap at (⅓)N turns of the first phasewinding 30. The second end of the grounding winding 44 is connected to atap at (⅓)N turns of the second phase winding 32. The second end of thegrounding winding 46 is connected to a tap at (⅓)N turns of the thirdphase winding 34. The grounding windings 42, 44, 46 should haveappropriate voltage phases. When a three-phase power fed to thedelta-connected windings is balanced, each of the voltages between eachof the phase lines a, b, c respectively and the neutral point n is thesame. The appropriate voltage phases of the grounding windings 42, 44,46 can be obtained by the same manner as that described in FIG. 3 andomitted herein.

Please refer to FIG. 8. FIG. 8 illustrates a diagram of disposing sixgrounding windings to the neutral-grounded structure for thedelta-connected windings of the present invention. In the presentinvention, six grounding windings can be disposed at most. Numbers ofthe six grounding windings are 48, 50, 52, 54, 56, 58. Each of thegrounding windings 48, 50, 52, 54, 56, 58 has (⅓)N turns. Theconnections of the grounding windings 48, 50, 52 are the same as thegrounding windings 42, 44, 46 shown in FIG. 7. That is, the first end ofthe grounding winding 48, the first end of the grounding winding 50, andthe first end of the grounding winding 52 are connected to a groundedpoint. The grounded point is a neutral point n. The second end of thegrounding winding 48 is connected to a tap at (⅓)N turns of the firstphase winding 30. The second end of the grounding winding 50 isconnected to a tap at (⅓)N turns of the second phase winding 32. Thesecond end of the grounding winding 52 is connected to a tap at (⅓)Nturns of the third phase winding 34. It is noted that the tap at (⅓)Nturns is counted from the connector X₁ to the connector X₃ (shown inFIG. 4). In contrast, another tap at (⅓)N turns can be counted fromconnector X₃ to connector X₁ (shown in FIG. 4). Accordingly, thegrounding winding 54 is disposed on another tap of the second phasewinding 32. The another tap at (⅓)N turns of the second phase winding 30is counted from the connector X₃ to the connector X₁ (shown in FIG. 4).The grounding winding 56 is disposed on another tap of the third phasewinding 34. The another tap is at (⅓)N turns of the third phase winding34 is counted from the connector X₃ to the connector X₁ (shown in FIG.4). The grounding winding 58 is disposed on another tap of the firstphase winding 30. The another tap at (⅓)N turns of the first phasewinding 30 is counted from the connector X₃ to the connector X₁ (shownin FIG. 4). The first end of the grounding winding 54, the first end ofthe grounding winding 56, and the first end of the grounding winding 58are connected to the neutral point n.

In the embodiment in FIG. 3, the second end of the grounding winding 34is connected to the tap of the first phase winding 30. In anotheraspect, the second end of the grounding winding 34 can be connected to atap of the second phase winding 32 (as the grounding winding 56 shown inFIG. 8). The tap at (⅔)N turns of the second phase winding 32 is countedfrom the connector X₁ to the connector X₃ (shown in FIG. 4). That is,the tap is the same as another tap at (⅓)N turns of the second phasewinding 32 which is counted from the connector X₃ to the connector X₁.If the voltage of the third phase winding 34 is Vca, the voltage of thegrounding winding 56 must be Vca. The voltage phase displacement betweenthe third phase winding 34 and the grounding winding 56 must be 180degrees.

The neutral-grounded structure for the delta-connected windings of thepresent invention can include at least one grounding winding and at mostsix grounding windings. That is, any one to five of the six groundingwindings 48, 50, 52, 54, 56, 58 in FIG. 8 can be removed. Only at leastone grounding winding can achieve the objective of the presentinvention.

Please refer to FIG. 9. FIG. 9 illustrates a voltage phasor diagramcorresponding to the neutral-grounded structure for the delta-connectedwindings in FIG. 8. There are six grounding windings 48, 50, 52, 54, 56,58 disposed on the phase windings 30, 32, 34 in FIG. 8. When athree-phase voltage fed to the phase windings is balanced, each of thevoltages between the phase lines a, b, c respectively and the neutralpoint n is the same.

The grounding winding 36 in FIG. 3 can be substituted by a groundingwinding group. Please refer to FIG. 10. FIG. 10 illustrates a diagram ofdisposing a grounding winding group on one phase winding. The firstphase winding 30, the second phase winding 32, and the third phasewinding 34 are delta-connected. Each of the first phase winding 30, thesecond phase winding 32, and the third phase winding 34 has a pluralityof turns, e.g. N turns. The grounding winding group 70 is disposed onthe third phase winding 34. The grounding winding group 70 includes twogrounding windings 60, 62 which are connected in parallel. Each of thegrounding windings 60, 62 has turns that are (⅓)N turns of the thirdphase winding 34. The first end of the grounding winding group 70 isconnected to a grounded point. The grounded point is a neutral point n.The second end of the grounding winding group 70 is connected to a tapof one of the other two phase windings 30, 32. The tap is at (⅓)N turnsof one of the other two phase windings 30, 32. In the presentembodiment, the second end of the grounding winding group 70 isconnected to a tap of the first phase winding 30. Of course, the secondend of the grounding winding group 70 can be connected to a tap at (⅔)Nturns of the second phase winding 32. The tap at (⅔)N turns of thesecond phase winding 32 is counted from the connector X₁ to theconnector X₃ (shown in FIG. 4), i.e., at (⅓)N turns counted from theconnector X₃ to the connector X₁ (shown in FIG. 4). If the groundingwinding group 70 is disposed on the second phase winding 32, thegrounding winding group 70 is similar to the grounding winding 56 inFIG. 8. In the present embodiment of FIG. 10, the voltage phase of thegrounding winding group 70 and the voltage phase of the third phasewinding 34 is out of phase. That is, if the voltage of the third phasewinding 34 is Vca, the voltage of the grounding winding group 70 must beVac so that each of the voltages between each of the phase lines a, b, crespectively and the neutral point n is the same. The above-mentionedterm “out of phase” means that a voltage phase displacement between thegrounding winding group 70 and the third phase winding 34 is 180degrees.

The grounding winding group can be disposed on two phase windings orthree phase windings as shown in FIG. 6 and FIG. 7. Six groundingwinding groups at most can be disposed as shown in FIG. 8. Theneutral-grounded structure for the delta-connected windings of thepresent invention can include at least one grounding winding group andat most six grounding winding groups. That is, any one to five of thesix grounding winding groups can be removed. Only at least one groundingwinding group can achieve the objective of the present invention.

The grounding winding group is not limited to two grounding windingwhich are connected in parallel. The grounding winding group can includethree or more grounding windings which are connected in parallel.

When one wire is connected to the neutral point n and grounded in one ofthe above-mentioned embodiments, the first phase winding, the secondphase winding, the third phase winding, and the wire are connected asthree-phase four-wire delta-connected windings. When no wire isconnected to the neutral point n in one of the above-mentionedembodiments, the first phase winding, the second phase winding, and thethird phase winding are connected as three-phase three-wiredelta-connected windings.

Please refer to FIG. 11. FIG. 11 illustrates a flow chart showing aneutral-grounded method for delta-connected windings. Thedelta-connected windings include a first phase winding, a second phasewinding, and a third phase winding. Each of the first phase winding, thesecond phase winding, and the third phase winding has a plurality ofturns, e.g. N turns. The neutral-grounded method includes the followingsteps. In Step 10, a grounding winding is disposed on at least one ofthe first phase winding, the second phase winding, and the third phasewinding. The grounding winding has turns that are (⅓)N turns of each ofthe phase windings. In Step 20, the first end of the grounding windingis connected a grounded point. The grounded point is a neutral point n.In Step 30, the second end of the grounding winding is connected to atap of one of the other two phase winding. The tap is at (⅓)N turns ofone of the other two phase windings. The voltage phase displacementbetween the grounding winding and the phase winding corresponding to thegrounding winding is 180 degrees.

According to the neutral-grounded structure for the delta-connectedwindings and method thereof, each of the first phase winding, the secondphase winding, and the third phase winding is a double-winding or amulti-winding.

Advantages of the present invention are described as following. Thefirst, when a three-phase power fed to the delta-connected windings isbalanced, each of the voltages between the phase lines a, b, crespectively and the neutral point n is the same. As a result,differences of each of the three phase-to-ground in the conventionalgrounding methods can be improved. The second, after the groundingwinding is disposed, dielectric strength of the delta-connected windingscan be increased. Accordingly, the delta-connected windings haveadvantages of both the delta-connected windings and the wye-connectedwindings.

While the preferred embodiments of the present invention have beenillustrated and described in detail, various modifications andalterations can be made by persons skilled in this art. The embodimentof the present invention is therefore described in an illustrative butnot restrictive sense. It is intended that the present invention shouldnot be limited to the particular forms as illustrated, and that allmodifications and alterations which maintain the spirit and realm of thepresent invention are within the scope as defined in the appendedclaims.

1. A neutral-grounded structure for delta-connected windings, thedelta-connected windings comprising a first phase winding, a secondphase winding, and a third phase winding, each of the first phasewinding, the second phase winding, and the third phase winding having Nturns, the neutral-grounded structure comprising: a grounding windingdisposed on at least one of the first phase winding, the second phasewinding, and the third phase winding, the grounding winding having turnsthat are (⅓)N turns of one of the phase windings, wherein the first endof the grounding winding is connected to the grounded point which isutilized as a neutral point, the second end of the grounding winding isconnected to a tap of one of the other two phase windings, the tap is at(⅓)N turns of one of the other two phase windings, and the voltage phasedisplacement between the grounding winding and the corresponding phasewinding is 180 degrees.
 2. The neutral-grounded structure fordelta-connected windings of claim 1, wherein each of the first phasewinding, the second phase winding, and the third phase winding is adouble-winding.
 3. The neutral-grounded structure for delta-connectedwindings of claim 1, wherein each of the first phase winding, the secondphase winding, and the third phase winding is a multi-winding.
 4. Theneutral-grounded structure for delta-connected windings of claim 1,wherein the tap is generated by drawing forth from (⅓)N turns of one ofthe other two phase windings.
 5. The neutral-grounded structure fordelta-connected windings of claim 1, wherein one wire is connected tothe neutral point, and the first phase winding, the second phasewinding, the third phase winding, and the wire are connected asthree-phase four-wire delta-connected windings.
 6. The neutral-groundedstructure for delta-connected windings of claim 1, wherein no wire isconnected to the neutral point, and the first phase winding, the secondphase winding, and the third phase winding are connected as three-phasethree-wire delta-connected windings.
 7. A neutral-grounded structure fordelta-connected windings, the delta-connected windings comprising afirst phase winding, a second phase winding, and a third phase winding,each of the first phase winding, the second phase winding, and the thirdphase winding having N turns, the neutral-grounded structure comprising:a grounding winding group disposed on at least one of the first phasewinding, the second phase winding, and the third phase winding, thegrounding winding group comprising at least two grounding windingsconnected in parallel, each of the grounding windings having turns thatare (⅓)N turns of one of the phase windings, wherein the first end ofthe grounding winding group is connected to a grounded point which isutilized as a neutral point, the second end of the grounding windinggroup is connected to a tap of one of the other two phase windings, thetap is at (⅓)N turns of one of the other two phase windings, and thevoltage phase displacement between the grounding winding group and thephase winding corresponding to the grounding winding is 180 degrees. 8.The neutral-grounded structure for delta-connected windings of claim 7,wherein each of the first phase winding, the second phase winding, andthe third phase winding is a double-winding.
 9. The neutral-groundedstructure for delta-connected windings of claim 7, wherein each of thefirst phase winding, the second phase winding, and the third phasewinding is a multi-winding.
 10. The neutral-grounded structure fordelta-connected windings of claim 7, wherein the tap is generated bydrawing forth from (⅓)N turns of one of the other two phase windings.11. The neutral-grounded structure for delta-connected windings of claim7, wherein one wire is connected to the neutral point, and the firstphase winding, the second phase winding, the third phase winding, andthe wire are connected as three-phase four-wire delta-connectedwindings.
 12. The neutral-grounded structure for delta-connectedwindings of claim 7, wherein no wire is connected to the neutral point,and the first phase winding, the second phase winding, and the thirdphase winding are connected as three-phase three-wire delta-connectedwindings.
 13. A neutral-grounded method for delta-connected windings,the delta-connected windings comprising a first phase winding, a secondphase winding, and a third phase winding, each of the first phasewinding, the second phase winding, and the third phase winding having Nturns, the neutral-grounded method comprising: disposing a groundingwinding on at least one of the first phase winding, the second phasewinding, and the third phase winding, and the grounding winding havingturns that are (⅓)N turns of one of the phase windings; connecting thefirst end of the grounding winding to a grounded point which is utilizedas a neutral point; and connecting the second end of the groundingwinding to a tap of one of the other two phase windings, and the tapbeing at (⅓)N turns of one of the other two phase windings, wherein thevoltage phase displacement between the grounding winding and the phasewinding corresponding to the grounding winding is 180 degrees.
 14. Theneutral-grounded structure for delta-connected windings of claim 13,wherein each of the first phase winding, the second phase winding, andthe third phase winding is a double-winding.
 15. The neutral-groundedstructure for delta-connected windings of claim 13, wherein each of thefirst phase winding, the second phase winding, and the third phasewinding is a multi-winding
 16. The neutral-grounded method fordelta-connected windings of claim 13, wherein the tap is generated bydrawing forth from (⅓)N turns of one of the other two phase windings.17. The neutral-grounded method for delta-connected windings of claim13, wherein one wire is connected to the neutral point, and the firstphase winding, the second phase winding, the third phase winding, andthe wire are connected as three-phase four-wire delta-connectedwindings.
 18. The neutral-grounded method for delta-connected windingsof claim 13, wherein no wire is connected to the neutral point, and thefirst phase winding, the second phase winding, and the third phasewinding are connected as three-phase three-wire delta-connectedwindings.